Atmospheric carbon-14 production from neutron leakage in fusion energy systems

Neutron-producing fusion systems can generate atmospheric carbon-14 when neutrons leak into nitrogen-containing gas. We use MCNP6.2 neutron-transport calculations to estimate the probability that leaked neutrons produce ${}^{14}$C through ${}^{14}$N$(n,p{)}^{14}$C under representative near-ground conditions. For 14.1 MeV deuterium-tritium source neutrons, the conversion probability is 0.25-0.50 across the geometries studied; softer leakage spectra can give larger yields. Scaling this response to a 1 GWe fusion plant shows that percent-level neutron leakage into air would produce an atmospheric ${}^{14}$C source within a factor of a few of natural global production. At a 2500 GWe fleet scale, limiting fusion-derived radiocarbon to 10% of the natural source implies a mean atmospheric leakage fraction of order ${10}^{-6}$. These results provide a screening-level source-term estimate for atmospheric ${}^{14}$C production from terminal neutron leakage in neutron-producing fusion systems, with particular relevance to architectures containing open ports, beamlines, ducts, or other streaming paths.